Elsevier

Physica B+C

Volume 111, Issues 2–3, November–December 1981, Pages 327-352
Physica B+C

Absolute intensities and perpendicular temperatures of supersonic beams of polyatomic gases

https://doi.org/10.1016/0378-4363(81)90112-1Get rights and content

Abstract

The centre-line intensity I(0)(s−1sr−1) of a supersonic beam source is fully described by the peaking factor κ=π(I(0)/N), with N (s−1) the flow rate. For a skimmerless source in a cryoexpansion chamber we find κ = 2.07, 1.48, 1.47, 1.20 and 1.17 for Ar, O2, N2, CO2 and CH4, respectively. These results are in fair agreement with numerical solutions of the flow field downstream of the sonic plane.

The shielding effect of the skimmer is conveniently described by mapping the perpendicular distribution, which cools geometrically, into a constant virtual source distribution in the nozzle plane. We use a bimodal distribution to analyze the beam profile measurements. For Ar and Kr the radius R1 of the narrow virtual source is given by R1/zref=4.4(Ξ/100)0.28 with zref=0.802Rn, Rn being the nozzle radius, Ξ=3.189((γ−1)/γ)12n0zref(C6/kT0)13the source parameter, n0 and T0 the number density and the temperature of the reservoir, respectively. These results are in good agreement with a “last collision surface” extension of the thermal conduction model by Habets. For O2 and N2 we find R1/zref=10.1(Ξ/100)0.42 with zref= 0.598Rn. By scaling the “last collision surface” from the Ar and Kr data for the case of diatomics we find fair agreement with a modified thermal conduction model by Klots.

The broad virtual source is very important for describing the shielding effect of the skimmer. The radius R2 is in the range 4⩽R1/R2⩽8 and its population consists of 30% (γ=53) to 50% (γ=75) of all particles. For Ξ⩽100 th e loss of intensity is less than 10% if we use the design rules Rs=20Rn for monoatomic and Rs=40Rn for diatomic gases, with Rs the radius of the circular skimmer opening.

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